Heat Engines Part VI - Unconventional Conventional Internal Combustion Engines

This final tutorial on innovative heat engines deals with significant modifications of established designs rather than entirely new art. For reasons made clear below, I believe that this is the area where growth is most likely.

Defining Conventional

A conventional internal combustion engine by our definition would be a reciprocating engine—that is, it would employ pistons. Rotary types, as we have seen, are not well established in any market other than in large scale power generation, and we see little likelihood of their supplanting reciprocating types in other applications.

Use of pistons still leaves the designer with considerable scope for innovation. He can utilize unusual mechanical linkages for converting the linear motion of the piston to rotary motion. He can utilize exotic materials. He can design the injectors in such a manner that the fluid dynamics of combustion are made more conducive to efficiency. He can design the engine around the use of unconventional fuels. And he can alter the method of ignition. Some designs, as we shall see, employ several such strategies.

The Surprising Return of the Two Stroke

The two stroke internal combustion engine was invented in the eighteen eighties by an English mechanical engineer named Dougald Clerk. Clerk’s original design was somewhat akin to what today is known as the stepped piston two stroke, but he felt that this first iteration, which utilized a displacer piston somewhat in the manner of the Stirling engine, was inelegant and overly complex, and in the following decade he developed the basic design that is still used today in the majority of two stroke production engines.

Clerk’s engine, and most of the two stroke designs that followed, hew to a simple basic plan. The intake stroke and the exhaust stroke of four cycle engines are eliminated as are intake and exhaust valves in the cylinder head. Instead the cylinder is provided with ports which are uncovered when the piston reaches the top of the cylinder. The cavity within the cylinder presents a partial vacuum in relationship to the crankcase, and a fuel-air mixture rushes in from the crankcase to fill the vacuum after the fumes of combustion have issued out of the exhaust port. Then the fuel air mixture is compressed on the downstroke, ignited—in most cases, by a conventional spark plug—and the piston is driven upward to produce mechanical energy.

Two stroke engines of the usual design have two singular virtues: mechanical simplicity and high power density. The first is obvious, and the second arises from the fact that they produce one power stroke per each revolution rather than one for every two revolutions as is the case with four stroke designs. Thus for a given horsepower rating, a two stroke engine is apt to be approximately half as massive as its four stroke counterpart. For this reason two strokes have long been favored in power tools, lawn mowers, small outboard engines for boats, snowmobiles, experimental aircraft—any application where weight is a crucial issue.

In automobiles and in larger stationary generators two strokes have not been favored, however, and today traditional designs are in increasing disfavor within most of their traditional markets. And the reason for that has to do with the inherent limitations of the standard design.

The biggest failure of the traditional two strokes is their inability to burn fuel cleanly and completely. Because the exhaust fumes are not pushed out of the cylinder by the action of the piston as is the case with four strokes, a good deal of exhaust gas remains and mingles with the incoming fuel air mixture. Worse yet, the fuel has to be mixed with lubricants because the air filled crankcase cannot be provided with a sump of motor oil, and thus the engine must depend upon the oil and gas mixture for lubrication. The presence of such lubricants in the gasoline along with the exhaust fumes lingering in the cylinder makes for very incomplete combustion, resulting in poor efficiency and copious emissions. Moreover, most two stroke engines release a lot of uncombusted oil into the environment.

Lubrication is predictably much less effective in two strokes than in four strokes, and thus they tend to run hot and are prone to failure if operated at peak power for any length of time. They are also difficult to muffle because the exhaust system demands a free flow of gases. For the same reason, turbo-charging is difficult to implement. Operators of jet skis and motocross motorcycles were willing to tolerate the limitations of two strokes because they put out so much power per unit of mass, but governmental agencies in charge of air and water quality have moved to limit or eliminate the use of two strokes in many areas of the world. The Republic of Singapore, for instance, has completely outlawed conventional two stroke engines within its jurisdiction.

Not surprisingly, manufactures of two stroke engines, facing the attrition of their core markets, began to get innovative. Most began to license one of two direct injection technologies that promised to ameliorate the incomplete combustion problem.

The remedies in common use today are the Orbital Engines and Ficht direct injection systems. Both retain the usual air intake method but inject fuel separately and in such a manner that the air fuel mixture is stratified and occupies only one portion of the cylinder cavity and does not mix with the exhaust fumes. This results in a much more complete combustion and sharply reduced emissions.

Other innovations have involved providing the exhaust system with an air pump to assist in clearing the cylinders, and what is known as active radical combustion, a type of compression ignition resulting in a cleaner burn and eliminating the need for spark plugs. Honda has actually manufactured combustion two strokes that exhibit active radical combustion.

The use of supercharging or turbo-charging can also considerably improve efficiency and reduce emission, largely by forcing exhausts gases out of the cylinder at the top of the power stroke.

Yet another interesting innovation is the stepped piston two stroke which utilizes a displacer piston which also serves to improve the breathing of the engine. The idea goes back to the eighteen eighties and has recently been revived by an English manufacturer named Bernard Hooper Engineering.

Advances in metallurgy have also led to improvements in two stroke performance and reliability. Hirth in Germany has succeeded in manufacturing highly reliable two stroke aircraft engines using exotic aluminum alloys that hold up under very high temperatures.

We have already mentioned two stroke diesels in a previous post. Huge examples of the latter known for some reason as “cathedral engines” have long been used in merchant ships, and recently a number of companies including Wilksch and Aero-Diesel in Germany and DeltaHawk in the U.S have developed smaller engines, most aimed at the general aviation market. The Aero-Diesel units are remarkable in achieving outputs in excess of one horsepower per pound, an impressive and unprecedented accomplishment in a compression ignition engine.

Finally, mention should be made of the Pivotal Engineering two stroke, a design from New Zealand originally developed for motorcycle racing. The Pivotal engine uses a most unusual piston which reciprocates on a pivot within a combustion chamber with curved walls, making the engine a sort of semi-rotary. The pivoting piston is hollow and provides for internal water cooling, addressing the heating problem that has plagued all two stroke designs. And because the motion of the piston is rotary, the mechanical linkages are considerably simpler than is the case for conventional piston engines. An Orbital Engine direct ignition system is utilized as well. Interestingly, this engine has been designed from the onset to run on hydrogen as well as gasoline.

Pivotal Engineering is relatively well financed compared to most startups in the ICE business, but like all others faces an uphill struggle to penetrate major markets. Like most other startups they are aiming at the stationary generation market.

The Four Stroke Sector

Four stroke piston engines represent very mature technology and of course also represent the incumbent technology in most markets. Space does not permit a discussion of every recent innovation in this area, but we can identify the most significant.

The most important development we see is the high pressure common rail injection system for compression ignition engines which is utilized in most automotive diesels manufactured in Europe today. High pressure common rail substitutes a single injection system for multiple injectors and results in higher power output and improved efficiency. Engines so equipped exhibit none of the typical sluggishness long associated with diesels.

Direct injection in spark ignition engines we have discussed previously. It permits double digit increases in fuel efficiency at no sacrifice of power. Currently direct injection is offered in one six cylinder engine designed by Mazda, but we expect it to become commonplace in the mid term.

Plasma ignition and catalytic ignition are interesting and promising innovations that are said to result in improved efficiency in gasoline engines. Currently catalytic ignition is confined to the aftermarket. SmartPlugs of Utah is the principal manufacturer. Plasma ignition is used in certain European exotic sports cars. Plasmatronics LLC, and Light Speed Engineering are two of the better known manufacturers.

Another invention that holds promise is the Dolphin ACI pulse jet supercharging system. This invention, which has performed well in independent tests, is a form of supercharging where the energy of the exhaust is utilized, as in a turbo-charger, but where a gating system provides the engine with successive pulses of pressurized air rather than continuous pressure. The system borrows the principal of operation used in the old German V1 pulse jet “buzz bomb”, and is claimed to provide greatly increased power and efficiency over turbo-charging along with reduced emissions.

The most interesting development we have seen in the four stroke realm is the Axial Vector engine, a thoroughly innovative product based on an old design known as the Scotch yoke engine. A Scotch yoke is a mechanical device invented by Leonardo da Vinci that directly translates linear motion into rotary motion without a conventional crankshaft. Scotch yoke engines are typically lighter, smaller, simpler, and more efficient than their conventional counterparts, but have suffered from limited operating lives due to stress induced failure of the Scotch yoke itself. Axial Vector claims to have solved those problems.

The Axial Vector design uses twelve cylinders and six double acting pistons, each of which communicates with two cylinders. The cylinders are disposed in two tubular arrangements and fire in sequence while driving a sinusoidal cam. The company claims that an engine weighing about sixty pounds is capable of outputting some five hundred horsepower without supercharging.

The engine purportedly utilizes plasma ignition, ceramic cylinders and cylinder sleeves, and a proprietary injection system with piezoelectric actuators and sensors for controlling droplet size. The system is computer controlled. The Axial Vector is said to be capable of running on any fuel without modification and of achieving 45% efficiency. Oil rather than water cooling is employed, contributing to a substantial weight savings. Tektronix is an investor.

The Axial Vector engine is based in part on a much older design known as the DynaCam which was developed for use in small aircraft in 1957 and has also been used in naval torpedoes. DynaCam, the company, sold intellectual property to Axial Vector and is now embroiled in a dispute with the latter while attempting to market the older design in the general aviation market.

The Axial Vector engine, at least on paper, is the most impressive new design I have seen, and the firm itself is well endowed with financial resources, a must in the business of new engine development where prototyping generally requires tens of millions of dollars minimally. The firm has come under fire from certain online financial reporting services, however. The company is targeting military transportation markets initially, a high risk strategy with high potential payoffs. Axial Vector itself reports having signed a contract with an Indian manufacturer who will actually produce the engines.

In Summation

Thus concludes our survey of innovation within the vast discipline of heat engine design. My point is simple. Exclusive concentration on fuel cells as a solution to energy problems seems to me unwarranted in view of continued advances in heat engine designs and the extreme difficulties in developing cost effective, long lasting fuel cells.